The present invention relates to a process for purifying a composition containing (meth)acrylic acid, an apparatus for producing pure (meth)acrylic acid, an apparatus for polymerizing (meth)acrylic acid, (meth)acrylic acid and polymers obtainable by these processes, their use and substances containing them.
“(Meth)acrylic acid” is used in this text for compounds having the nomenclature names “methacrylic acid” and “acrylic acid”. Of the two compounds, acrylic acid is preferred according to the present invention.
It is often desirable to work up (meth)acrylic acid to high purities of at least 99.9% by weight for their use in polymers. In the hygiene industry, for example, superabsorber polymers based on polyacrylates may contain specific by-products only below the detection limit.
Crystallization is mentioned as an alternative to the production of high-purity organic substances. The two particular processes, which are used industrially, are suspension crystallization and layer crystallization (Wintermantel et al, Chem. Ing. Tech. 1991, 63, 881-891; Steiner et al, Chem. Ing. Tech. 1985, 57, 91-102).
However, a crystallization step alone is often insufficient to remove by-products sufficiently well from the crystals as microinclusions of mother liquors or the incorporation of impurities at crystal defects, etc. cannot be ruled out under finite crystal growth conditions. The adhesion of mother liquor to the crystal can also impair the purity of the products.
For this reason, the crystals produced, particularly in the case of a crystal suspension, are frequently washed with washing fluids after separation from the mother liquor and/or the crystals are subjected, during layer or suspension crystallization, to an exudation process during which impurities of any type may optionally be depleted. A process of this type may be carried out continuously in what are known as wash columns. The dissertation by Poschmann (Zur Suspensionskristallisation organischer Schmelzen und Nachbehandlung der Kristalle durch Schwitzen und Waschen, Diss. Uni. Bremen, Shaker Verlag Aachen 1996) provides an overview.
EP 0616998 discloses a process for producing at least 99.9% by weight acrylic acid starting from previously purified product having an acrylic acid content of 97.771% by weight. The effect of purification is achieved by the cooperation of dynamic and static layer crystallization processes. What is known as falling film crystallization is adopted as the final means of crystallization. An apparatus of this type may only be operated discontinuously and necessitates complex apparatus and logistics and comparatively high-energy consumption owing to the many process cycles required to obtain the necessary purities.
It is disclosed in WO 99/14181, to crystallize crude (meth)acrylic acid for purification in a first step and to work it up in a second step, optionally using wash columns. The process disclosed therein starts directly with the condensation products of catalytic gaseous phase oxidation for the production of (meth)acrylic acid. With this process, it is disclosed that the mother liquor produced after washing and separating the crystals is recycled into the condensation stage. With this process, a product having a purity of 98.8816% by weight was obtained from 90.972% by weight acrylic acid. However, this is not sufficient for some industrial applications. For example, the content of inhibitors and aldehydes in the pure (meth)acrylic acid is a critical variable, which, if exceeded, may lead to drawbacks, for example in the subsequent polymerization process.
Nienrood et al have disclosed how acrylic acid may be well purified by suspension crystallization and subsequent treatment in a hydraulic wash column (so-called TNO process; Proc. Bremer International Workshop on Industrial Crystallization, Bremen, 1994, Ed.: J. Ulrich, p. 4-11; Purification Potential of Suspension Growth Melt Crystallization, Proc. 4th International Workshop on Crystal Growth of Organic Materials, Bremen, 1997, Ed.: J. Ulrich, Aachen Shaker Verlag, p. 139-145). The acrylic acid used in these experiments was obtained from Aldrich and had a purity of 99.75% by weight. It could be purified to a purity of 99.97% by weight by this process. However, the use of lower-purity, acrylic acid has not been disclosed.
With former processes for producing (meth)acrylic acid, the (meth)acrylic acid obtained in a (meth)acrylic acid reactor and subsequently dissolved in water in a quench absorber is usually subjected to a complex distillation process, which sometimes necessitates the use of entrainers such as toluene so that high degrees of purity are then obtained by a crystallization process. Distillation processes for (meth)acrylic acid have the drawback that the (meth)acrylic acid is exposed to heat during the distillation process, through which its properties change, for example due to partial (pre)polymerization.